To be suitable for inflating crash restraint systems, a propellant composition must meet a unique and exceptionally demanding set of physical and chemical criteria. It must be resistant to aging, automobile environments, and be adaptable to assembly line handling. The propellant must produce non-toxic decomposition products upon ignition and burn at a sufficiently low temperature to prevent harm to the driver. The composition must provide a wide margin of safety during assembly, use and disassembly of the automobile. The propellant must burn at a rapid and highly reproducable rate to act as a satisfactory inflation means. The properties required of an inflation propellant are discussed in greater detail below.
The first parameter considered in the selection of an inflator propellant is that of which gas to produce. The gas must be non-toxic and should not react with environmental substances in manners leading to the production of toxic or dangerous materials. Gases suitable for use include nitrogen, carbon dioxide, and the noble or inert gases. No satisfactory means for generating noble gases exists at the present time. Carbon dioxide is usually generated by burning carbon containing compounds in the presence of an oxidizing agent. Present art devices for generating carbon dioxide sufficiently rapidly for use as inflator propellants however produce gas temperatures considered excessive for use in air bags. The use of organic compounds can also lead to production of carbon monoxide, carbon dioxide or other toxic by-products. Practical inflator propellants therefore rely on nitrogen as the reaction product. Nitrogen is commonly produced by decomposition of an azide.
The second parameter considered is the burn rate of the propellant. A crash restraint device must inflate within 0.1 second of an impact to be effective. Obviously the propellant must completely burn in less than this amount of time. The propellant must not however burn so rapidly as to create a danger of explosion. Burn rate is determined by chemical composition and physical configuration. An alkali metal azide decomposes too slowly at obtainable temperatures to be useful alone as a propellant. Oxidizers are added to the azide to increase burn rate. Oxidizers that have been used include metallic chlorates, perchlorates, oxides, and nitrates. Metallic chlorates find limited use as their sensitivity to shock and friction presents a possibility of preignition and possible malfunction. In general, perchlorates and oxides are the most satisfactory oxidants. Perchlorates as oxidants in combination with azides have the advantage of rapid burning and the disadvantage of a higher temperature of decomposition. Oxide oxidants, in general, burn more slowly and at lower temperatures.
The third parameter considered is the temperature of decomposition. If the propellant decomposes at too high a temperature the hot gas produced could constitute a danger to the user. This parameter must be balanced with the burn rate as rapid burning compositions most often have a high temperature of decomposition. One attempted solution to this problem has been to add a coolant. Organic compounds are commonly used for this purpose, but often result in the production of carbon monoxide upon reaction with an oxidant that may be present. Florocompounds have also been used with similar difficulties. The most satisfactory propellant considering the first three parameters would thus appear to be fast burning metallic oxide oxidizer and an azide.
A propellant composition which comes close to satisfying the first three parameters is sodium azide and iron oxide. One such composition uses 70% by weight sodium azide and 30% by weight iron oxide. The characteristics of this propellant are influenced to a large extent by the form of iron oxide used in formulation. While all iron oxide used is described by the chemical formula Fe.sub.2 O.sub.3 there are marked variations in burn rate and handling properties dependent upon the particle size and crystalline form. This propellant is compounded by either dry mixing of commercial ferric oxide and sodium azide or wet mixing of commercial ferric oxide with sodium azide in the presence of water to partially dissolve the sodium azide and insure a more homogeneous mix. The water does not react chemically with either of the constituents. This composition exhibits a satisfactory burn temperature and does not produce toxic products but is plagued by insufficient physical strength and a slow burn rate. Research on improving this propellant has led to an improvement in a fourth relevant parameter.
The fourth parameter, strength, is related to the physical form of the propellant. As stated above in the discussion of burn rate the physical form of the propellant influences the burn time. Since inflator propellants burn very rapidly rather than detonate the burn time is directly dependent upon the surface area exposed at any given time during the propellants' decomposition. Propellant shapes that have been used include powders, granules and shaped pellets of various types. Powdered propellants burn rapidly but tend to separate upon vibration and storage into their constituent parts. Powders can also clog filtering means in the gas stream. Granules and pellets burn at slower rates and do not separate. Pellets have the most reproducable burning rates as their shape is fixed by their method of manufacture. A satisfactory propellant thus must be capable of pelletization. Once a propellant is pelletized it is essential that the pellets formed are not prone to breakage or cracking. If propellants of a given burn rate and given shape are broken in any significant numbers the surface area is increased. If the surface area of a pellet is increased the burn time decreases. Too rapid a burn time can lead to explosion or destruction of the inflator apparatus. Cracks in propellant pellets have a similar effect and are to be avoided. Prior art metal oxide-azide propellants have been prone to breakage and cracking as well as having insufficient burn rate.
To summarize, the parameters governing satisfactory propellants are: gas produced, burn rate, temperature of decomposition and strength. No one of the prior art propellants satisfy all of these parameters.